DENVER - Mitt Romney's presidential campaign has been savaging what it calls President Barack Obama's "unhealthy" obsession with "green jobs."

The Republican challenger criticizes the government program that propped up solar manufacturer Solyndra, and he mocks Obama's vision of a boom in employment, citing a European study to argue that new solar or wind-energy positions would destroy jobs elsewhere.

But when a campaign spokesman said last week that Congress should let a tax break for wind energy producers expire at the end of the year, some Republicans were concerned the candidate had gone too far.

Republican Rep. Tom Latham, R-Iowa, noting that nearly 7,000 Iowans work in the wind industry, assailed the Romney campaign for "a lack of full understanding of how important the wind energy tax credit is for Iowa and our nation." Iowa's senior senator, Chuck Grassley, told reporters he didn't believe Romney really opposed the extension, and he joined five other GOP lawmakers in voting for it in the Senate Finance Committee.

The Obama campaign quickly organized conference calls for reporters and circulated fact sheets showing the deep support the credit has in such swing-voting states as Iowa, Colorado and Nevada.

Obama will appear in Denver and western Colorado Wednesday to promote his economic plan, and the wind tax credit may well come up.

The backlash on the wind tax issue shows the risks Romney takes in targeting a fast-growing and popular industry that Obama has embraced. However, Romney's aides argue the campaign is just making a principled economic argument against excessive government interference in the marketplace - one that the conservative movement, which Romney has struggled to win over, has praised.

Indeed, Patrick Hedger, a researcher at FreedomWorks, a small-government group that is a prominent backer of the tea party movement, called Romney's position "a happy surprise." He added that Republicans who feared a political cost from Romney's position were stuck in an outdated way of thinking. "We've got to get out of this cycle of buying votes with money we don't have," Hedger said.

But critics contend that Romney, who counts members of the fossil fuels industry as major financial supporters and relies on the head of an oil company as his energy adviser, has backed himself into a corner. "I think it's really a knee-jerk reaction to what this president has done," said Jeff Gohringer, a spokesman for the League of Conservation Voters. "He (Romney) is actually going to states and advocating cutting thousands of their jobs."

Surveys show the industry's popularity. A Gallup poll in March found Americans nearly twice as likely to favor wind and solar energy as coal or oil. The American Wind Energy Association released a poll last month showing that more than half of Iowa's voters say they would not back a presidential candidate who did not support expanding wind power. A January poll by Colorado College found that a majority of voters in six Western states believe that expanding renewable energy will create more jobs.

In Colorado, GOP Rep. Doug Lamborn says he was pleased the Romney campaign took a stand against the tax credit. "It shows he's standing on principle and not pandering to win votes," Lamborn said. But Lamborn is the only one of the state's seven congressional representatives to oppose the extension.

Obama made green jobs a focus of his 2008 campaign, and he included tens of billions of dollars in incentives to promote energy efficiency and the renewable industry in federal stimulus efforts. After Solyndra's bankruptcy last year, Republicans lined up to criticize the administration program that guaranteed the firm's loans, and Romney has broadened the attack to the administration's support for the entire industry, even in states where it is popular.

During a May stop in Colorado, where the poll from Colorado College found two-thirds of residents believe renewable energy will create jobs, Romney mocked Obama for spending billions to create "green jobs." He asked the crowd: "Have you seen those jobs anywhere?"

The Romney campaign argues that what it calls "much-ballyhooed" wind and solar jobs may actually lessen the total number of jobs available because they replace positions in dirtier, but more labor-intensive industries. It cites a controversial Spanish study that found that every renewable energy job in that country destroyed 2.2 others.

"In place of real energy, Obama has focused on an imaginary world where government-subsidized windmills and solar panels could power the economy," Romney wrote in a March op-ed piece published in The Columbus Dispatch in Ohio. "This vision has failed."

The Obama campaign has been eager to respond, especially after an Iowa Romney campaign spokesman, Shawn McCoy, said last week that the candidate favored doing away with the wind tax credit.

"Mitt Romney would slash investments in clean energy, which would cede leadership of these critical sectors of our economy to competitors like China and India - and the jobs that go with them," the Obama campaign said in a statement, adding that the Republican has opposed ending tax breaks for the oil industry.

The American Wind Energy Association estimates that 37,000 jobs would be lost if the tax credit isn't extended this year. The credit was created in a 1992 energy bill signed by President George H.W. Bush and was renewed in a 2005 measure that passed a Republican Congress and was signed by President George W. Bush.

The Obama administration gave Romney an opening by overselling the promise of renewable energy jobs, said Jonathan Rothwell, a senior associate at the Brookings Institution who helped write a report on the growth of green jobs. The jobs will come, Rothwell said, but it will probably take a couple of decades. "The Obama administration did exaggerate the short-term benefits of the green economy by implying it would drive us out of the recession," he said.

Rothwell noted that renewable energy jobs are well-dispersed across the country and, while small in number, are growing rapidly. "That explains why there is broad, and in some cases bipartisan support," he said.

The issue came up in an annual green energy conference hosted by Senate Majority Leader Harry Reid on Tuesday in Las Vegas.